Correct Answer: B. Competitive inhibitor of DHFR
Methotrexate (MTX) is a folate antagonist that acts as a competitive inhibitor of dihydrofolate reductase (DHFR), the enzyme responsible for converting dihydrofolate to tetrahydrofolate (THF). THF is the critical one-carbon donor required for both purine and pyrimidine synthesis. By blocking DHFR, MTX depletes the intracellular THF pool, thereby inhibiting both nucleotide synthesis pathways simultaneously. This dual blockade makes MTX particularly effective in osteosarcoma, where rapidly dividing malignant cells have high nucleotide demands. In the Indian context, MTX remains a cornerstone of the neoadjuvant and adjuvant chemotherapy regimen for osteosarcoma (per AIIMS/NIMS protocols), often combined with cisplatin and doxorubicin. The mechanism is not that MTX itself is a nucleotide analog—rather, it prevents the synthesis of the cofactor (THF) needed to manufacture nucleotides. High-dose MTX with leucovorin rescue is standard in Indian tertiary cancer centers, exploiting the fact that malignant cells accumulate MTX more readily than normal cells and have reduced capacity for leucovorin rescue.
Why the other options are wrong
A. Analog of UMP — This is wrong because MTX is not a nucleotide analog. It does not directly mimic uridine monophosphate or any nucleotide. Rather, it is a folate analog that inhibits the enzyme (DHFR) needed to generate the cofactor for nucleotide synthesis. Nucleotide analogs like 5-FU or gemcitabine work differently—they are incorporated into DNA/RNA or directly inhibit nucleotide synthases. This option confuses the class of anticancer drugs and is a common NBE trap for students who conflate all antimetabolites. C. Blocks purine synthesis — This is partially true but incomplete and therefore wrong as the best answer. MTX does block purine synthesis, but only indirectly by depleting THF. However, it blocks both purine and pyrimidine synthesis equally, not purine alone. The question asks for the mechanism of action, which is DHFR inhibition—the upstream cause. Selecting this option shows incomplete understanding of the biochemical hierarchy and misses the discriminating mechanism that defines MTX's action. D. Blocks pyrimidine synthesis — Like option C, this is partially true but incomplete. MTX does impair pyrimidine synthesis via THF depletion, but it equally impairs purine synthesis. More critically, this option describes a downstream effect, not the primary mechanism. The question specifically asks for the mechanism of action—the direct molecular target. DHFR inhibition is the mechanism; nucleotide synthesis blockade is the consequence. This trap lures students who know MTX affects nucleotide synthesis but haven't internalized the biochemical hierarchy.
High-Yield Facts
- DHFR inhibition by MTX depletes tetrahydrofolate (THF), the one-carbon donor for both purine and pyrimidine synthesis.
- High-dose MTX (≥1 g/m²) with leucovorin rescue is standard in Indian osteosarcoma protocols; leucovorin bypasses DHFR to replenish THF in normal cells.
- MTX is a folate antagonist, not a nucleotide analog; it blocks the cofactor, not the substrate.
- Osteosarcoma chemotherapy in India typically uses MTX + cisplatin + doxorubicin (MAP regimen) in neoadjuvant and adjuvant settings.
- Selectivity for malignant cells occurs because tumor cells accumulate MTX via reduced folate carrier (RFC) and have lower dihydrofolate polyglutamylase activity, trapping MTX intracellularly.
Mnemonics
MTX → DHFR → THF → Nucleotides MTX blocks DHFR → THF depleted → both purines and pyrimidines cannot be made → cell death. Remember: MTX is a folate antagonist, not a nucleotide analog. Use this when you see 'methotrexate mechanism' in any cancer or rheumatology question. Folate Antagonist vs. Nucleotide Analog Folate antagonists (MTX, trimethoprim) block the enzyme that makes the cofactor. Nucleotide analogs (5-FU, gemcitabine, 6-MP) mimic or block synthesis of the nucleotide itself. MTX = upstream (enzyme), analogs = downstream (substrate). Discriminates MTX from other antimetabolites.
NBE Trap
NBE pairs MTX with 'blocks purine/pyrimidine synthesis' to trap students who know MTX affects nucleotide synthesis but confuse the mechanism (DHFR inhibition) with the consequence (nucleotide depletion). The question asks for mechanism, not effect.
Clinical Pearl
In Indian cancer centers, high-dose MTX is given with leucovorin rescue 24–48 hours later. Leucovorin (folinic acid) is a reduced folate that bypasses DHFR, rescuing normal tissues while malignant cells (which accumulate MTX and have lower rescue capacity) remain vulnerable. This is why understanding DHFR inhibition—not just 'nucleotide blockade'—is clinically critical for dosing and toxicity management.
_Reference: KD Tripathi Pharmacology Ch. 10 (Anticancer Drugs); Robbins Ch. 7 (Cell Injury & Adaptation—nucleotide synthesis); Harrison Ch. 89 (Osteosarcoma & chemotherapy)_